1. Field of the Invention
The present invention relates to novel compositions based on rare earth sulfides, and, more especially, to compositions based on rare earth sesquisulfides and comprising alkali and/or alkaline earth metal values, and having, inter alia, improved chromatic properties.
As utilized herein, by the term "rare earth elements" are intended the elements belonging to the lanthanide group of the Periodic Table having an atomic number of from 57 to 71, as well as yttrium which has an atomic number of 39.
2. Description of the Prior Art
The rare earth sesquisulfides, having the general formula M.sub.2 S.sub.3, in which M represents at least one rare earth, are compounds per se known to this art and are widely described in the literature, patent and otherwise.
It too is known to this art, inter alia, that such rare earth sesquisulfides are useful pigment colorants for various substrates, such as, for example, plastics, paints and others. They are also used as detectors for the presence of sulfur, or for the production of materials which are optical windows, both for visible light as well as for the infrared.
By way of example, the use of rare earth sesquisulfides as pigment colorants has been described, in particular in EP-A-0,203,838, assigned to the assignee hereof and hereby expressly incorporated by reference.
For this particular use, it is found that the rare earth sesquisulfides to date known to this art have properties, in particular chromatic properties, which remain inadequate for the broad spectrum of such applications.
Another difficulty resides in the processing required for the synthesis of this type of compound.
The majority of processes for preparing monophase rare earth sesquisulfides of high purity entail solid/gas-type reactions.
Thus, these sulfides may be prepared by reacting hydrogen sulfide with a rare earth oxide at high temperature (see again EP-A-0,203,838) or under high pressure. The reaction of a sulfurizing agent, such as sulfur, H.sub.2 S and/or CS.sub.2, with a rare earth compound such as, for example, an oxide, a carbonate or an oxalate has also been described. Processes of this type are described, in particular, in FR-A-2,100,551, or in the article by Heindl and Loriers, Bulletin de la Societe Chimique de France, No. 303 (1974).
Nonetheless, the aforesaid prior art processes present the disadvantage in that they are not easily carried out, or cannot be carried out at all, on an industrial scale because they require either the use of H.sub.2 S to produce a rare earth sesquisulfide free from oxysulfide values or temperature and/or pressure conditions incompatible with industrial production. Moreover, they are difficult to control and the parasitic or competing formation, together with the rare earth sesquisulfide having the desired crystalline phase, of a rare earth oxysulfide is difficult to prevent. Thus, the products obtained have inadequate chromatic coordinates and, moreover, the original color of the sesquisulfide may be degraded by, for example, a rare earth oxysulfide which does not have the desired color.